As is known, with a heat storage adsorbent material prepared by mixing a heat storage material containing heat storage capsules (referred to as “microcapsules” hereinafter) comprising a phase change material absorbing/discharging latent heat in accordance with change in temperature, the phase change material being charged in outer shells and an adsorbent material, when temperature change occurs in repetition as the adsorbent material adsorbs an adsorption target substance to generate heat or desorbs the target substance to absorb heat, the temperature change is minimized by the heat storing function of the heat storage material, thus preventing reduction in the adsorbing/desorbing performance of the adsorbent material.
There is known an adsorbent canister using such heat storage adsorbent material charged in a case, the canister being provided for e.g. preventing evaporable fuel (organic solvent) such as gasoline supplied to an internal combustion engine of an automobile or the like from being discharged to the outside (e.g. into the atmosphere). With this adsorbent canister in operation, e.g. when the vehicle is stopped, excess evaporable fuel is adsorbed to the adsorbent material within the canister and when e.g. the vehicle runs, ambient air is introduced as a “purge gas” into the case to desorb the evaporable fuel, so that this fuel is recycled to the internal combustion engine for instance. Therefore, with the heat storage material comprising heat storage adsorbent material charged within a case, there is the risk of the phase change material contained in the case leaking to the outside of the outer shell as a result of the evaporable fuel if the adsorption target substance destroys or permeates the outer shell of the heat storage capsule. The breakage or damage of the outer shell and the resultant leakage of the phase change material contained therein to the outside can occur also, due to degradation of the outer shell of the heat storage capsule by moisture or water content present within this case. Such leakage of phase change material to the outside can lead to deterioration in the heat storing function and reduction in the adsorption/desorption performance.
In order to solve the above problem, according to a technique disclosed in Japanese Patent Application “Kokai” No. 2008-069680 (JP '680), the outer shell of the heat storage capsule charged with a phase change material is comprised of an inner layer made of a hydrophobic resin and an outer layer made of a hydrophilic resin. Further, these heat storage capsules are formed into a molded heat storage material with a binder of e.g. thermosetting type resin and then this molded heat storage material is mixed with a molded adsorbent material formed separately, thus forming a heat storage adsorbent material. And this resultant heat storage adsorbent material is charged in a canister for use.
With this heat storage adsorbent material, the heat storage capsule employs an outer shell having the two-layered construction consisting of the inner layer and the outer layer. Therefore, even when an evaporable fuel such as an ethanol mixed gasoline (ethanol 10%, gasoline 90%) is used, it is possible to effectively restrict or avoid permeation of this evaporable fuel through the outer shell of the heat storage capsule, so that the adsorption/desorption performance can be maintained favorably for an extended period of time. Further, since the heat storage capsules are formed into the molded heat storage material (corresponding to “molded heat storage material” of the present invention) with using a binder (corresponding to “molding binder” of the present invention) in advance and then mixed with a separately molded adsorbent material, it is possible to minimize destruction of the heat storage capsules having a relatively low strength in the course of mixing with this molded adsorbent material, thus preventing reduction in the heat storing function. As a result, it is said that deterioration in the adsorption/desorption performance of the heat storage adsorbent material can be prevented.
However, with the heat storage capsules of the molded heat storage material constituting the heat storage adsorbent material disclosed in JP '680, the outer layer of the outer shell is made of a hydrophilic resin. So, there is the risk of this capsule being dissolved by moisture or the like. Further, as it is difficult for this resin to completely cover the inner layer constituting the outer shell, with contact with the evaporable fuel, there occurs progressive destruction of the outer shell of the heat storage capsule (especially, its outer layer), whereby the phase change material contained therein may leak to the outside.
Further, for example, in JP '680, the heat storage capsules are formed with the binder into the molded heat storage material. This forming is done with mixing a relatively small amount of binder (e.g. 5 weight parts of binder relative to 95 weight parts of heat storage capsules). The larger the amount of binder, the higher the ratio of this binder having no heat storing ability, hence, the lower the heat storing performance per unit volume. This is the probable reason why the amount of binder is kept small. In this respect, with such molded heat storage material molded with mixing of a relatively small amount of binder, as shown in SEM (scanning electron microscope) photos in FIGS. 8 (a) and (b), it may be understood that on its surface and the inner side, the periphery of mutually contacting portions of the plurality of substantially spherical heat storage capsules is bound with the binder. Therefore, although the material is molded with the binder, gaps or voids are formed in the other portions on the surface and the inner side of the molded heat storage material than those where the heat storage capsules are present. With the presence of such gaps or voids in the surface and the inner side, if the capsules are exposed to an evaporable fuel such as gasoline, in particular, a mixture of alcohol and gasoline which is highly aggressive or disintegrative to the heat storage capsules, this mixture will readily soak into the molded heat storage material, thus tending to invite progressive destruction of the outer shells of the heat storage capsules. As a result, the phase change material contained therein may leak to the outside.
And, such leakage of the phase change material leads to deterioration in the heat storing performance and deterioration in the adsorption/desorption performance.